Synthesis and evaluation of tribo-mechanical and corrosion properties of electroless Ni-P coating on AZ91D Mg alloy with stannate conversion underlayer

The work reports on synthesis and comprehensive evaluation of different properties of a bilayer protective coating on Mg alloy of AZ91D grade. Initially the alloy substrates were treated in stannate bath for 15, 30, 45, 60, 75 and 90 min for synthesizing an MgSnO₃.3H₂O layer. Electroless Ni-P was then deposited for 30 and 60 min, on the stannate layer synthesized for 60 min. A time-dependent cyclic growth behaviour was observed for stannate layers, characterized by formation and dissolution of MgSnO₃.3H₂O globules. The stannate coatings displayed polycrystalline structure. Hardness and scratch resistance were strongly influenced by uniformity of the coatings. While the uncoated alloy exhibited hardness of only 93 HV_0.05, a remarkable improvement in hardness to 484 HV_0.05 was obtained after 60 min of stannate treatment. High critical load of adhesion (Lc₃) of 29 N was also obtained for the same. The stannate layers exhibited coefficient of friction (CoF) of 0.4–0.6 as compared to 0.7–0.8 for bare alloy. The wear loss also notably reduced. In tribo-test, hard and well-adherent coating restricted surface deformation under action of normal load. This in-turn, reduced mechanical engaging at interface, leading to low CoF and wear loss. The dense stannate coating also reduced corrosion rate by 19 times by hindering penetration of corrosive medium. Electroless deposition for 30 min on stannate conversion layer yielded only sporadic growth of Ni-P, which improved to a dense, uniform layer after 60 min of deposition. The Ni-P top coating aided the stannate underlayer in furthering the tribo-mechanical and corrosion characteristics.